So, a new bike, Avid Elixir 5 180mm front and rear. My second ride was on a short dirt road, 340m denivelation.

I held the brakes (mainly the front) more than 50% of the time, as the road was too steep for my skills.

At the end of the descent, there was slight smell of burnt plastic, coming from the front calliper.

What has caused this and is it at reason for concern?

On another account, a couple of Tektro Augura smelled severely of burnt plastic at the middle of the descent (another bike, another rider). She finished the descent and has not seen ill effect since (well, the front braking is not as good as before, but no idea if this is since that happening or later).

Used to be, if you left the emergency brake set while driving in a car you'd get a "rubbber burning" smell, and a similar smell braking continuously on a long downhill. (I think they changed the composition of the pads so this is less pronounced anymore.) I'd not be surprised that bicycle disk brakes produce some sort of "burning" smell. But certainly it's possible to overheat the brakes and cause problems other than just the smell.
–
Daniel R HicksNov 22 '13 at 16:31

Even when you already have your disks broken in, long drag braking can cause them to overheat and fade. This leads to a kind of vicious cycle in which the brake is less effective and you start using it even more.

Brakes are designed to cool themselves with the available airflow, even a few seconds every minute can make a difference. Of course, that's easier said than done when you feel like you are over your head. Braking effectively is a skill that takes practice. If you are dragging the brakes constantly to control your speed, you'll be better off coming to a complete stop and regrouping.

Be sure that before riding on new brakes you are "bedding" them. This process allows for residue from your pads to contact the new resign-less rotors, creating an effective breaking surface. Without doing this, you are likely to introduce pad residue in a feathered pattern that will produce squeals. Also the bedding smooths out your pad surface (they start a little rough).

You bed your pads simply by consistent, hard breaking. Do this before you get on the trail. When I build a bike up to sell, I immediately take it to the parking lot to check suspension, shifting, and to bed the pads on the rotors. Get up to speed and pull both front a rear levers while shifting your weight behind the saddle!

With all that said you are getting that smell because the high amount of heat generated by the fiction caused by breaking. The smell is fine. Just make sure you break them in consistently. Proper bedding will reduce heat (and smell) by creating more consistent and powerful breaking.

Technical detail: The pads are a "composite" of particles or fibers (ceramic, copper, steel, et al) held together with a plastic-like material. Just about any of the plastic-like materials will smell when sufficiently hot, especially when new. This odor, in itself, is not (necessarily) symptomatic of a problem.
–
Daniel R HicksNov 24 '13 at 14:29

At the shop, they said this was normal (duhhhhh!). In the end of any DH the brakes will smell of burnt plastic, they said.

If there is a problem with pad glazing, it will be felt on the levers, making them noticeably stiffer, and the braking force reduced, they said. If the was air on the system, the levers are again the part to look at - a sudden lack of braking, followed by gradual (temporary) recovery, they said.